Many applications exist for methods to measure and identify unknown materials or substances as well as for portable measurement devices embodying such methods. Such applications include, for example, field identification of unknown substances by law enforcement, hazardous materials personnel, fire personnel and security personnel, as well as detection of prohibited substances at airports and in other secure and/or public locations, and identification of pharmaceutical agents, industrial chemicals, explosives, energetic materials, and other agents. Such field identification situations can include those where it is desirable to obtain such identification information under time critical situations. Also, to be useful in a variety of situations, it is advantageous for the portable measurement devices to have a handheld form factor and to rapidly provide accurate results.
In certain embodiments, the measurement devices and methods provide for contact between a sample of interest and the measurement device via a prism that is, for example, positioned in a protrusion of the measurement device's enclosure. The prism, which can be formed from a material such as diamond or Zinc Selenide, operates by ensuring that non-absorbed incident radiation is directed to a detector after undergoing total internal reflection within the prism. As a result, reflected radiation is coupled with high efficiency to the detector, ensuring sensitive operation of the measurement devices.
Samples of interest can be identified based on the reflected radiation that is measured by the detector. The reflected radiation can be used to derive infrared absorption information (e.g., absorption spectrum) corresponding to the sample, and the sample can be identified by comparing the infrared absorption information to reference information that is stored in the measurement device. In addition to the identity of the sample, the measurement device can provide one or more metrics (e.g., numerical results) that indicate how closely an infrared absorption spectrum or information matches the reference information. Further, the measurement device can compare the identity of the sample of interest to a list of prohibited substances—also stored within the measurement device—to determine whether particular precautions should be taken in handling the substance, and whether additional actions by security personnel or the like, for example, are warranted. A wide variety of different samples can be interrogated, including for example solids, liquids, gels, powders, and various mixtures of two or more substances.
When measuring the infrared absorption spectrum of a sample of interest using a conventional technique; a background or reference spectrum without the presence of the sample is typically taken close in time to acquiring the spectral information for the sample. The spectral data for the background or reference spectrum is taken so as to minimize environmental and/or instrument effects on the absorption spectrum being acquired.
The process for acquiring a background or reference spectrum typically involves performing or acquiring N scans of spectral data. As illustration, the process for certain measurement devices involves taking about 8 scans to arrive at a background or reference spectrum with an acceptable signal to noise ratio. This is not limiting as about 64 scans can be taken to establish a background spectrum, such as when the measurement device is being used under laboratory conditions.
Using techniques well known in the art (e.g., Fourier transform), the acquired spectral data is processed and combined so as to yield a background or reference spectrum. This background or reference spectrum is then used in combination with a subsequently acquired spectrum for a sample of interest to determine the spectral output or spectrum that is associated with the sample. This process of establishing a background or reference spectrum every time a sample is to be analyzed, however, may not be beneficial in some applications, such as when time is of the essence in determining the absorption spectrum associated with the sample, this process increases the time required to determine the make-up of the specific sample being analyzed.
One specific analysis technique that is used is generally referred to as Fourier Transform Infrared (FTIR) Spectroscopy which obtains infrared spectrum of absorption, emission or Raman scattering of a solid, liquid or gas. In this process a Fourier transform (a mathematical process) is used to convert the raw data into the actual spectrum. In use, an FTIR spectrometer simultaneously collects spectral data in a wide spectral range.
In FTIR spectroscopy, rather than shining a monochromatic beam of IR light at the sample, a beam containing many frequencies of infrared light shines on the sample, and one measures how much of that broadband beam is being absorbed (e.g., absorbed by the sample). Next, the beam is modified to contain a different combination of frequencies, giving a second data point. This process can be repeated as many times as desired. Afterwards, a computer deconvolves the acquired spectral data to infer what the absorption is at each wavelength. In other words, the computer (e.g., digital processor) processes the data so to yield a spectrum representative of the sample of interest.
The beam described above is generated by starting with a broadband light source—one containing the full spectrum of wavelengths (e.g., IR wavelengths) to be measured. The light shines into a Michelson interferometer—a certain configuration of mirrors, one of which is moved by a motor. As this mirror moves, each wavelength of light in the beam is periodically blocked, transmitted, blocked, transmitted, by the interferometer, due to wave interference. In this way, different wavelengths are modulated at different rates, so that at each moment, the beam coming out of the interferometer has a different spectral output.
Each such movement of the mirror generally corresponds to a scan and is generally considered the process of acquiring spectral data associated with one movement of the mirror. As indicated above, the absorption spectrum associated with the sample or the background results from a computer or other digital processing device/apparatus, taking all the acquired spectral data and processing it so as to infer what the absorption is at each wavelength.
There is found in U.S. Pat. No. 8,248,588 (owned by the assignee of the present invention), an apparatus that includes: (a) an enclosure including an aperture; (b) a prism mounted in the enclosure so that a surface of the prism is exposed through the aperture; (c) an optical assembly contained within the enclosure, the optical assembly including a radiation source and a radiation detector, the source being configured to direct radiation towards the prism and the detector being configured to detect radiation from the source reflected from the exposed surface of the prism; and (d) an electronic processor (e.g., digital processor) contained within the enclosure, the electronic processor being in communication with the detector. This apparatus can be configured so that, during operation, the electronic processor determines information about a sample placed in contact with the exposed surface of the prism based on radiation reflected from the exposed prism surface while it is in contact with the sample. Such an apparatus also routinely performs a process that yields a background or reference spectrum from N scans (e.g., 8 scans) as described herein, which process is undertaken close in time to the taking of the sample spectrum.
It thus would be desirable to provide new methods, devices and/or apparatuses that would be capable of reducing the frequency of taking background or reference spectra such as when using FTIR spectroscopy or FTIR-ATR spectroscopy. It would be particularly desirable to provide such a device and method that could be used as a portable chemical identification analyzer in the hot zone of an emergency response while not requiring the background or reference spectrum to be taken close in time to the taking of the sample spectrum. It also would be particularly desirable to perform checks prior to acquisition of the sample spectra which can be used to assess the ATR interface when using FTIR-ATR spectroscopy techniques, as well as compensating for changing conditions whenever possible.